Process for the preparation of polyphosphates of organic bases

ABSTRACT

A simple and economical process for the preparation of polyphosphates of organic bases consists of reacting a mixture of phosphorus pentoxide and at least one organic nitrogen base with at least one compound which releases water accompanied by decomposition under the prevailing conditions in such a molar ratio that upon decomposition of the water-releasing compound at most essentially 2 mol. water are produced per mol. phosphorus pentoxide. Thus-prepared polyphosphates are particularly suitable as flame-protection agents for plastics.

This application is a 371/DE 02/03065 filed on Aug. 22, 2002.

Polyphosphates of organic nitrogen bases such as guanidine and melamineare becoming increasingly important as flame-protection agents inplastics and as a flame-retardant finish for textiles. Polyphosphatemixtures used as flame-protection agents for plastics are intended todecompose as close as possible to the decomposition point of theplastic, as too-low decomposition points already lead to decompositionand thus to the formation of steam and undesired blistering in theplastics product during the preparation of the mixture of plastic andflame-protection agent. To avoid such undesirably low decompositionpoints of the flame-protection agent, the polyphosphate mixtures shouldcontain no orthophosphate if at all possible and where necessary also nopyrophosphate.

Various process types for the preparation of polyphosphates of organicbases are known to date, however these known processes all have specificdisadvantages.

A process type for the preparation of polyphosphates of organic basesconsists of firstly preparing an orthophosphate of the organic base andthen converting same into polyphosphates of the organic base by heatingto a temperature over 300° C. This process is described for example inWO 97/44 377. The required relatively high temperatures of this processtype make the process uneconomical if it is considered in particularthat flame-protection agents are mass-produced products. According toU.S. Pat. No. 6,114,421, specific organic nitrogen bases are reactedwith phosphoric anhydride in specific concentrations and under specificreaction conditions.

Another process type consists of neutralizing polyphosphoric acidcustomary in the trade with the organic base. Polyphosphoric acidscustomary in the trade are however mixtures which contain ortho- andpyrophosphoric acid so that the product mixtures contain the undesiredortho- and pyrophosphate of the organic base. In addition, thepolyphosphoric acids customary in the trade are viscous to solid anddifficult to process. The ortho- and pyrophosphates contained areconverted into polyphosphates only at temperatures above 306° C. whichresults in the same disadvantage as the first process type describedabove.

A third process type starts with alkaline polyphosphate or alkalinepyrophosphate which are dissolved in water and acidified with HCl. Toremove the chloride ions, the products must be washed and dried, whichmakes the process time-consuming and laborious. When used asflame-protection agents, the flame-protection agent must not containchloride ions as they can lead to harmful by-products.

The object of the invention was thus to create a simple and economicalprocess for the preparation of polyphosphates of organic bases whileavoiding the disadvantages of the state of the art, the products beingintended to be as free as possible from orthophosphates and wherenecessary also from pyrophosphates.

The process according to the invention which achieves this objectconsists of reacting a mixture of phosphorus pentoxide and at least oneorganic nitrogen base with at least one compound which releases wateraccompanied by decomposition under the reaction conditions in such amolar ratio that during decomposition of the water-releasing compound atmost essentially 2 mol. water are produced per mol. phosphoruspentoxide. Polyphosphates within the meaning of the invention arecondensed phosphates with a chain length of at least 2.

Surprisingly the problem of the state of the art of distributing therequired water quantity homogeneously and uniformly in the mixture ofphosphorus pentoxide and base is thus solved. The required quantity ofwater is distributed completely homogeneously and uniformly in thecustomarily introduced mixture of phosphorus pentoxide and organicnitrogen base so that a local oversupply of water which leads to theformation of ortho- and pyrophosphoric acid is avoided.

Regardless of the preparation process, the object of the invention isachieved by a mixture of polyphosphates of organic nitrogen bases whichhave a) a weight loss upon heating to 320° C. of less than 2 wt.-%, b) apH value of a 10-% aqueous suspension at 25° C. of >5, preferably of 5.2to 7.7, particularly from 5.8 to 7.0; and c) a solubility in water at25° C. of less than 0.1, preferably less than 0.01 g/100 ml water.

Such polyphosphate mixtures can be expediently prepared according to theprocess according to the invention described above.

There can be considered in principle as water-releasing compounds allthose which decompose under the selected reaction conditions accompaniedby the formation of water, such as for example substances containingcrystallization water, borax, aluminium hydroxide or magnesiumhydroxide, which however can be used only if the decomposition productsof these compounds do not interfere with or are even advantageous in thepolyphosphate mixtures when used. Preferably used as water-releasingcompounds are those which, in addition to water, release only volatiledecomposition products, preferably oxalic acid in anhydrous form or asdihyrdrate or formic acid which upon decomposition, in addition towater, produce only carbon dioxide and carbon monoxide which leave thereaction mixture on account of their volatility.

The molar ratios in the reaction mixture are to be set according to theabove statements such that at most approx. 2 mol. water are produced permol. P₂O₅ in the reaction mixture. It is to be taken into account that 1mol. oxalic acid (anhydrous) produces 1 mol. water upon decompositionand oxalic acid dihydrate produces 3 mol. water. In the preparation ofpyrophosphate, the starting substances are used in such a molar ratiothat essentially 2 mol. water are produced per mol. P₂O₅ in thewater-releasing compound. In the preparation of long-chainedpolyphosphates, this value is approx. 1 mol. water per mol. P₂O₅. Whenthe term “approx.” or “essentially” is used in this description, thevalue is usually ±10, preferably ±5% of the value given.

As organic nitrogen base there can be used for this purpose a per-seknown nitrogen base such as for example polyvinylamine,polyethyleneimine, piperazine, methylenediamine, melamine, guanidine,methylolmelamine or their condensates and also their mixtures. Melamineand guanidine are preferred. Assuming that the nitrogen bases have abase radical, the molar ratio is to be set such that if pyrophosphate isobtained as desired and the pyrophosphate is completely saturated withbase radicals, essentially 2 mol. base must be used per P atom in theproduct. If long-chained polyphosphates are intentionally obtained, themolar ratio of base:P used is to be essentially 1:1 (base: P₂O₅essentially 2:1). By setting the molar ratio, the saturation with baseradicals can be varied.

The reaction according to the invention can take place at ambienttemperature, however it is too slow for practical purposes as a rule. Itis therefore preferred to carry out the reaction at an increasedtemperature, in particular in the range from 100 to 250° C., preferably180 to 250° C., particularly preferably 200 to 220° C. The preferredminimum temperature of 200° C. is above the melting temperature ofoxalic acid, which has a positive effect on the reaction of thecomponents. On the other hand, the temperatures to be applied are wellbelow the temperatures of over 300° C. required with the knownprocesses. This makes the process economical, bearing in mind that as aresult of the homogeneous distribution of the water in the reactionmixture the formation of orthophosphate and where necessarypyrophosphate is avoided.

The procedure for the process according to the invention is preferablythat the required quantity of phosphorus pentoxide is mixed with theorganic nitrogen base, the mixture is taken to the desired temperatureand the required quantity of water-releasing compound is then added andhomogeneously distributed in the reaction mixture by mixing.

The polyphosphate mixtures according to the invention are advantageouslyused as flame-protection agents for plastics, preferably thermoplasts,in particular polyamides and polyesters.

The following examples serve to further explain the invention.

EXAMPLE 1

An oil-heated double-Z kneader with an effective volume of 5 l ischarged with 1250 g (10 mol.) melamine and 710 g (5 mol.) phosphoruspentoxide. The mixture is homogenized and heated to a temperature of220° C.

Within 15 minutes 450 g (5 mol.) of anhydrous oxalic acid are then addedand mixed in. After the addition, there is a secondary reaction time of15 min before the melamine polyphosphate produced is removed from thekneader.

EXAMPLE 2

In the same kneader as described in Example 1, 1260 g (10 mol.) melamineand 710 g (5 mol.) phosphorus pentoxide are mixed and heated to 220° C.As in Example 1, however, 900 g (10 mol.) anhydrous oxalic acid areintroduced within 15 min. After a further 15 min at 220° C., puredimelamine pyrophosphate is removed from the kneader.

EXAMPLE 3

3780 g melamine (30 mol.) are mixed with 2130 g (15 mol.) phosphoruspentoxide in a laboratory ploughshare mixer with an effective volume of10 l and thermal oil-heated double jacket and heated to a temperature of200° C. Within 30 min, 630 g (5 mol.) oxalic dihydrate are then added.After a secondary reaction time of a further 15 min, the melaminepolyphosphate formed was removed from the mixer.

EXAMPLE 4

In the same mixture as described in Example 3, 5400 g (30 mol.)guanidine carbonate are mixed with 2130 g (15 mol.) phosphorus pentoxideand heated to a temperature of 250° C. 1350 g (15 mol.) anhydrous oxalicacid are added in metered doses over a period of 1 h. After a furtherhour's secondary reaction time, the guanidine polyphosphate formed isremoved from the mixer.

EXAMPLE 5

In a kneader according to Examples 1 and 2, 861 g (10 mol.) piperazineare mixed with 710 g (5 mol.) phosphorus pentoxide and heated to 100° C.Within 15 min, 210 g (1.7 mol.) oxalic dihydrate are then added. Thesecondary reaction time is 1 h before the piperazine polyphosphateformed is removed.

1. A process for preparing polyphosphates of organic bases comprising reacting phosphorus pentoxide, at least one organic nitrogen base selected from the group consisting of piperazine, methylenediamine, melamine, guanidine, methylolmelamine or condensates and mixtures thereof, and at least one water releasing compound which releases water accompanied by decomposition under the reaction conditions in such a molar ratio that upon decomposition of the water-releasing compound, at most essentially 2 mol. water are produced per mol. phosphorus pentoxide.
 2. A process as claimed in claim 1 in which said at least one water-releasing compound comprises a water of hydration-containing compound selected from the group consisting of borax, aluminum hydroxide, magnesium hydroxide, and mixtures thereof.
 3. A process as claimed in claim 1 in which said reaction is carried out at a temperature of from about 100 to about 250° C.
 4. A process as claimed in claim 1 in which said reaction is carried out at a temperature of from about 180 to about 250° C.
 5. A process as claimed in claim 1 in which said reaction is carried out at a temperature of from about 200 to about 220° C.
 6. A process as claimed in claim 1 in which said at least one water-releasing compound comprises oxalic acid anhydrate, oxalic acid dihydrate, or formic acid. 